The invention relates to a process for the separation or purification, in one or more steps, of molten, liquid, or dissolved compounds or mixtures by fractional crystallization and to an apparatus for performing said process. The process and apparatus of the invention enable the economical separation of multicomponent materials on an industrial scale. Multiple-step fractional crystallization is carried out in accordance with the following flow diagram, which shows a five-step process. ##STR1##
The process has been suggested and used of feeding the material to a middle step having the same concentration as the material fed. This type of operation results in a mother liquor of an eutectic system having a specially low concentration of desired product, as best the eutectic concentration. For two component systems with solid solution over the whole range of concentration, the pure component can be separated out at the first and last steps, respectively, provided there are a sufficient number of steps.
Theoretical and experimental investigations have shown the actual separation by crystallization to depend on the rate of crystallization and on the mass-diffusion in the stationary or laminar boundary layer between the solid and liquid phase, as expressed by the following equation: ##EQU1## Where k.sub.o =best possible distribution coefficient with the extremely slow growth of a single crystal
k=actual distribution coeffficient with practical rates of crystallization PA0 f=rate of crystallization PA0 .delta.=thickness of boundary layer, where molecular diffusion takes place PA0 D=coefficient of molecular diffusion of the component in the fluid boundary layer.
Hitherto, the equation led workers in the art to reduce the boundary layer thickness .delta. and to adapt the crystallization rate f to the individual separation problem by promoting natural, or by inducing forced, convection.
It is known that the separation effect in each step of a multi-step crystallization is greatly influenced by the degree to which the mother liquor can be removed from the crystals, the problem arising of removing as much as possible of the mother liquir trapped between crystals or by capillary forces or held on the surface of the crystal layer.
It has been observed that at very high rates of crystallization an unstable supercooling can occur at the crystal-liquor interphase, resulting in a dendritic crystal formation which unfavorably influences separation. It has been proposed to prevent the appearance of this formation by inducing a steady flow of heat, with the attendant temperature gradient, from the liquor to the crystal layer.
It has also been proposed to form the crystals on cooled inside surfaces of tubes, with the liquor flowing at high speed and therefore turbulently, so as to obtain a high value of the actual distribution coefficient k. Further, it is common practice with tube ice manufacturing to feed the water to be frozen as a liquid film falling down on the inside of cooled metal tubes, without, however, the object of separation. The formed ice tubes are removed by melting a thin layer of ice at the boundary between the metal tube and the ice by applying the necessary heat through the metal tubes. The loosened ice tube fall into an ice container.
Also proposed has been to produce pure and very fine salt crystals on the inner surface of vertical tubes, by streaming the dissolved salt as a film down the inner surface while evaporating the solvent. For the formation of a single homogenous crystal a process is known (temperature sequence process for growth of large crystals) by which the number of nucleii is reduced by control of temperature and growth within the crystal forming area.